Just wanted to add that Whole Foods has a lot of great muffin recipes on their website, and may have the exact one your son likes. I make their Morning Glory muffins and freeze them. A few seconds in the micro or a couple minutes in the oven to warm up, and they are great.

This is a double recipe (so you can freeze to have on hand) that makes at least 36 medium muffins. (When my kids were toddlers, I made a single recipe and used a mini-muffin tin.) You can play around with whatever veggies you want to slip in there, of course.